Double-acting servomotor

ABSTRACT

Double-acting servomotor comprising a cylinder and a piston system with piston rods, wherein the clearance extending along the surfaces of the piston system and said cylinder movable relatively to each other and connecting two working chambers is divided into two divisional clearances lying in line and being separated by a circular groove, said divisional clearances being at its one end wider than at its other end and the circular groove being connected with a space in which in operation the pressure has such a value that the pressure at the smaller ends of the divisional clearances is always lower than the pressure at the wider ends of said divisional clearances.

United States Patent 1191 Blok et al. Dec. 11, 1973 [54] DOUBLE-ACTING SERVOMOTOR 3,003,471 10/1961 Bodem et al 92/86 X 3,035,879 5/1962 Hann et a1. 92/162 X [76] Inventors: Coymanstraaf 3,058,450 10/1962 Lissatl 1. 92/162 Mqerkapelle; f? Jan 3,063,423 11 1962 Riordan 92/86 x Ju ana aan Punacker, both of 3,522,760 8/1970 Putman 92 127 Netherlands Primar Examiner-4min C. Cohen 22 F1 d: F b.4 1971 y 1 1 e e AttorneySnyder and Butrum [21] Appl. No.: 112,679

[57] ABSTRACT [30] Foreign Application Priority Data Double-acting servomotor comprising a cylinder and a Feb. 5, 1970 Netherlands 7001609 Piston System with Piston rods, wherein the clearance extending along the surfaces of the piston system and 52 U.S. c1 91/431, 91/466, 92/127, Said cylinder movable relatively to each other and 92/162 connecting two working chambers is divided into two 5 m CL 1 11/08, F16], 13/04, 0 31/00 divisional clearances lying in line and being separated 58 Field Of Search 92/86, 127, 151, by a Circular groove, Said divisional clearances being 92/162, 166, 86.5191/466, 431; 308/D1G. 1 t its one end wider than at its other end and the circular groove being connected with a space in which in 5 References Cited operation the pressure has such a value that the pres- UNITED STATES PATENTS sure at the smaller ends of the divisional clearances is 3 221 770 2/1965 Fa sandi 137/596 3 always lower than the pressure at the wider ends of 1 er 2,103,252 12/1937 Gartin 92/151 x Send dmslonal clearances 2,623,501 12/1952 Audemar 92/127 11 Claims, 4 Drawing Figures minnow 1 1915 3.777. 621

SiEEI 1 BF 2 IN VENTOR DOUBLE-ACTING SERVOMOTOR The invention relates to a double acting servomotor actuated by a pressure fluid and comprising a cylinder and a piston system with piston rods, said piston system and cylinder being able to make a movement relatively to each other and said cylinder comprising two working chambers, each of said chambers being delimited as its one end by a movable piston surface and at its other end by an end wall rigidly connected to the cylinder, said end wall provided with a passage for a piston rod, said two chambers each being connected via a conduit with a valve device which controls the supply of pres sure fluid to said working chambers. It is an object of the invention to provide such a servomotor which works with a very low friction.

In accordance with the invention in such a servomotor the clearance extending along the surfaces of said piston system and said cylinder movable relatively to each other and connecting the one of said working chambers with the other of said working chambers is divided into two divisonal clearances lying in line, said divisional clearances being separated by a circular groove, each of said divisional clearances being at its one end wider than at its other end whereas the total clearance at its outer ends has the same width, the circular groove being connected with a space in which in operation the pressure has such a value that the pres sure at the smaller ends of the divisional clearances is always lower than the pressure at the wider ends of said divisional clearances, and the clearances extending along the surfaces of the parts of the working chambers movable relatively to each other and connecting the working chambers with the atmosphere have a width which decreases from the inside to the outside. With such a servomotor it is possible to have a completely hydrostatic bearing of the piston system in relation to the cylinder. In all clearances between the moving parts, the width of the clearance in the direction of the low pressure end is decreasing. This has for effect that the pressure fluid exerts a centering action on the moving parts relatively to each other, so that these moving parts need not to contact each other. The so-called dry or coulomb friction is completely eliminated so that even very slow motions of the servomotor can be completely uniform. This is of special importance in the case that such servomotors are applied for driving a simulator. A further advantage is that no wear of the moving parts will occur anymore.

In an advantageous embodiment according to the invention the piston system consists of two pistons spaced apart and being connected by a piston rod passing through an intermediate wall separating the working chambers and forming part of the cylinder wall, the passage for the piston rod having at its ends a larger diameter than in the middle between its ends and the passage at its smaller diameter being provided with a circular groove which is connected with a space in which in operation the pressure is lowered than the pressure in the working chambers and the conduit for the supply of the pressure fluid open into the working chambers on both sides of the intermediate wall in the immediate vicinity of said wall. In such an embodiment of the servomotor, one has the advantage that also in the case that the servomotor is making a very large stroke, the conduit for the supply of the pressure fluid can be very short. Further it is an advantage that the discharge of the pressure fluid to the space of low pressure can occur through a fixed not-moving part if the cylinder is used as the stationary part of the motor.

According to the invention an extra conduit for the supply of pressure fluid opens into each working chamber, said conduit being provided with a restriction for allowing to pass an amount of fluid corresponding with the amount of fluid leaking away through the clearances. In the case that the normal supply of pressure fluid is closed in a position of rest of the piston, through these extra conduit is supplied such an amount of pressure fluid that the pressure in the working chambers is maintained, with the result that the hydrostatic bearing remains functioning.

The compensation of the leakages of fluid can also be obtained when in the servomotor according to the invention the piston system consists of two pistons spaced apart and being connected by a piston rod passing through an intermediate wall separating the working chambers and forming part of the cylinder wall, the passage for the piston rod having at its ends a smaller diameter than in the middle between its ends and the passage provided at its larger diameter with a circular groove which is connected with a space in which in operation the pressure is higher than the pressure in the working chambers and the conduit for the supply of the pressure fluid open into the working chambers on both sides of the intermediate wall in the immediate vicinity of said wall. In such an embodiment upon closing the main supply of pressure fluid to the working chambers, pressure fluid will leak through the clearances along the piston rod to the working chambers and along the pistons the pressure fluid will leak away to the outside. The compensation of the leakage is obtained without supplying extra means.

According to the invention the working chambers can be separated by a piston having at its ends a larger diameter than in the middle and being provided at its smaller diameter with a circular groove which is connected with a space in which in operation the pressure is higher than the pressure in the working chambers. Thepressure fluid leaking into the working chambers along the piston can escape through the clearances along the piston rod. Also in this embodiment of the servomotor an automatical compensation of the leakage is obtained without applying extra means.

Finally according to the invention the space in which the lower pressure is obtained is connected with the atmosphere.

The invention will be elucidated in the following description of an embodiment shown in the drawing.

In the drawing show:

FIG. 1 is a servomotor according to the invention in a longitudinal section.

FIG. 2 a longitudinal section according to FIG. 1 of a servomotor in another embodiment,

FIG. 3 a detail from FIG. 2 in another embodiment FIG. 4 a detail from FIG. 1 in another embodiment.

The servomotor shown in FIG. 1 consists of a cylinder 1 with a piston system movable in said cylinder 1 and comprising a piston 2 and piston rod 3 and 4. The piston rod 3 at the end of the cylinder 1 passes through a closure member 5 that is provided with a part 6 for guiding the piston rod. The part 7 is provided with an O-ring seal 8 and a device 9 for stripping dirt. The O -ring seal causes very little friction as the space 11 to be sealed is not under pressure. The part 6 has a conical shape so that the clearance 10 at the end directed to the piston 2 is wider than at the other end. The clearance opens with its narrow end into an annular groove 11 which is connected via a conduit 12 with a conduit 13 which opens into a reservoir 14.

In a corresponding way at the other end of the cylinder is arranged a closure member 15 which is provided with sealing rings and the part for guiding the piston rod has a clearance 16, the width of said clarance decreasing in the direction of the end directed away from the piston. At the narrow end of the clearance 16 is also an annular groove 17 which is connected with a conduit 18 which also opens into the reservoir 14. The piston 2 has a double conical shape so that the width of the clearance 19 and 20 between the piston 2 and the cylinder 1 decreases from the ends to the middle. The clearances 19 and 20 open at the middle of the piston into a circular groove 21 in the piston, which groove 21 is connected via a canal 22 and a flexible duct 23 with the reservoir 14.

Into the working chambers 24 and 25 of the cylinder 1 open ducts 26 and 27. The ducts 26 and 27 are connected with a valve 28. The valve 28 is supplied with pressure fluid through the conduit 29 and this fluid is supplied from the reservoir 14 via a duct 30. By means of a pump 31 the fluid is presurized and supplied to a reservoir 32 which is connected with the duct 29. The pressure in this duct 29 can be controlled by means of a pressure control valve. The valve 28 is controlled in a way not shown. The pressure fluid can be applied to the working chamber 24 or 25 at will. The pressure fluid which is discharged from one of the working chambers 24, 25 flows through the duct 53 back to the reservoir 14.

As a result of the described shape of the clearances 10 and 16, and 19 and 20 a centering action is exerted on the moving parts, so that a metallic contact is prevented. Independently of the proportion of the pressures in the working chambers 24 and 25 in relation to each other always the pressure at the wider end of the clearances 19 and 20 is higher than the pressure at the narrow end as the pressure fluid through the groove 21 and the duct 23 is discharged to the atmosphere so that at this end of the clearances there can be no pressure.

With the duct 29 are further connected the ducts 34 and 35. The duct 34, which contains a restriction 36 opens into the duct 27. The duct 35 contains a corresponding restriction 37 whereas this duct 35 opens into the duct 26. In the case that the valve 28 closes the supply to both working chambers as is possible in practice, there would be the risk that through the clearances 10, 16, 19 and 20 such an amount of fluid would leak away from the working chambers 24 and 25, that these working.chambers become pressureless. Then no pressure fluid is flowing through the clearances and the hydrostatic bearing is not functioning. Under these conditions a metallic contact will occur. The restrictions 36 and 37 are dimensioned in such a way that always through the ducts 34 and 35 such an amount of pressure-fluid is flowing to the chambers 24 and 25 that the leakage losses through the clearances are compen sated.

The increasing or decreasing width of the clearances is obtained by means of the conical shape. It is also possible to apply a stepped shape, thus the width of the clearances is changing discontinuously. Nevertheless the same centering action is obtained.

In the device described above, the pressure fluid from the annular groove 21 is discharged through the moving end of the piston rod 3. In the case that the piston 2 has to execute a very large stroke, the cylinder 1 must be very long, so that also the ducts 26 and 27 become very long. In combination with the compressibility of the pressure fluid this can be a disadvantage. In the embodiment shown in FIG. 2 this disadvantage is eliminated. The servomotor according to FIG. 2 consists ofa cylinder 40 in which two pistons 41 and 42 can move. The pistons 41 and 42 are connected by a piston rod 43. A member 44 connected with the piston 41 is functioning s piston rod and passes to the outside of the cylinder. The working chamber 45 and 46 in the cylinder are separated by a part 47 which is fixedly arranged in the cylinder 40. The piston rod 43 passes through a passage in the part 47. The part 47 has a double conical shape so that clearances 48 and 49 exist, the width of these clearances decreases from the end adjacent the working chambers to the other end. At the narrow end the clearances 48 and 49 open into a groove 50 which is connected through a canal 51 in a way not shown with a pressureless reservoir. Around the cylinder are mounted sleeves 52. The space inside the sleeves 52 is connected with the space at the pressure less side of the I pistons 41 and 42. The pressureless space 53 behind the piston 2 for example is connected through the duct 54 with the space 55 inside the sleeve 52. The space 55 is through the conduit 56 also connected to the pressureless duct 51. In a way not shown through the valve 57 pressure fluid can be supplied to the ducts 58 or 59. These ducts 58 and 59 open into the working chambers 45 and 46 on both sides of the part 47. Independently of the magnitude of the stroke of the pistons 41 and 42 the ducts 58 and 59 can be very short. The discharge of the pressureless fluid through the duct 51 takes place only through stationary parts. The centering action on the moving parts in relation to each other is obtained in the same way as is described for the embodiment according to FIG. 1. In the embodiment according to FIG. 2 the ducts 60 and 61 contain restrictions 62 and 63. Through these ducts 60 and 61 in the same way as is indicated in FIG. 1 for the ducts 34 and 35 the leakage through the clearances can be compensated.

In FIG. 3 a part 47 with the piston rod 43 passing therethrough is shown in another embodiment. From a circular groove 64 in the middle of the part 47 to both sides clearance 65 and 66 with decreasing width are extending. The groove 64 is connected through a duct 67 with a source for high pressure, for example the pump for the pressure fluid.

This has for result that the pressure in the groove 64 is always higher than the pressures in the working chambers 45 and 46. The ducts 56 for discharging the pressure fluid, which is leaking along the pistons 41 and 42, are similarly to the embodiment in FIG. 2 connected with a pressureless space. In the embodiment according to FIG. 3 the pressure fluid leaking to the inside through the clearances 65 and 66 can leak to the outside through the clearances along the pistons 41 and 42. In such a way there is an automatic compensation of the leakage losses and in the case that the servo valve 57 closes the supply of pressure fluid to the working chambers 45 and 46, the leakage flow is maintained so that the hydrostatic bearings remain functioning. In the embodiment according to FIG. 3 there is no need of an independent supply of pressure fluid through the ducts 60 and 61.

In FIG. 4 is shown a further embodiment of the piston 2 from FIG. 1. In this embodiment the piston 2 has its smallest diameter in the middle. The annular space 38 is connected with the duct 29 through a duct 39. In a similar way as described for the embodiment according to FIG. 3 pressure fluid of a higher pressure than the pressures in the working chambers 25 and 24 is supplied to the annular groove 38. The pressure fluid leaking along the piston 2 can leak away through the clearances and 16. So there is also an automatic compensation of the leakage losses so that there is no need of the ducts 34 and 35.

For the sake of clearness the clearances in the drawing are shown'exaggeratedly. The width of the clearances is of the magnitude of some hundredth of millimeters. The difference in width of one and the same clearance is also of the magnitude of hundredth of millimeters. As a result of the measures according to the invention the friction can be reduced so far that with a servo motor according to the invention one can perform almost completely uniform motions. The accelerations with such motions are less than 0,01 g.

What we claim is: i

l. A double acting servomotor comprising, in combination:

an elongate cylinder having a bore of uniform diameter;

piston means in said cylinder for defining first and second chambers therewithin, said piston means comprising a first piston member of frusto-conical shape having its larger end defining a restricted annular clearance space with said bore, a second piston member of frusto-conical shape having its larger end defining a restricted annular clearance space with said bore, and means joining said piston members to each other for movement in unison; a source of pressurized actuating fluid; control valve means selectively connecting said source to said chambers for differentially pressurizing them selectively to move the piston members in opposite directions and for holding the pressures in said chambers to hold said piston members in selected fixed positions;

bleed means defining a fluid circulation flow path through each chamber and past each associated piston member for continuously conveying actuating fluid past each piston member from the small end to the large end thereof and to a bleed flow exhaust means and including means for continuously supplying actuating fluid to each flow path independently of said control valve means.

2. A double acting servomotor as defined in claim 1 wherein said piston members are joined together with their larger ends in opposition and close proximity to define an annular space therebetween which is common to both said flow paths.

3. A double acting servomotor as defined in claim 1 wherein said piston members are joined together with their smaller ends in opposition and in close proximity to each other to define an annular space therebetween which is common to both said flow paths.

4. A double acting servomotor ay defined in claim 1 6 closing said bore between said piston members and having a double conical bore receiving said piston rod, said double conical bore having a central annular space which is common to both of said flow paths.

5. A double acting servomotor as defined in claim 2 wherein said bleed means includes a conduit leading to said annular space between the piston members for bleeding off actuating fluid therefrom and said means for continuously supplying actuating fluid including a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers and each having a restriction therein for metering the bleed flows of actuating fluid to such chambers.

6. A double acting servomotor as defined in claim 3 wherein said bleed means includes a conduit leading to said annular space between the piston members for supplying actuating fluid thereto and a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers.

7. A double acting servomotor as defined in claim 4 wherein said double conical bore tapers away from both piston members, and said bleed means includes a conduit leading to the central annular space of said wall member to supply actuating fluid thereto and a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers for metering the bleed flows of actuating fluid from such chamber.

8. A double acting servomotor as defined in claim 4 wherein said double conical bore tapers outwardly toward both piston members, and said bleed means includes a conduit leading to the central annular space of said wall member to bleed actuating fluid therefrom and a pair of conduits one connected to one of chambers and the other connected to the other of said chambers and each having a restriction therein for metering the bleed flow of actuating fluid to such chambers.

9. A double acting servomotor as defined in claim 1 including at least one piston rod joined to said first piston member and projecting axially of said bore in the cylinder to pass outwardly therefrom, said cylinder having an end wall provided with :a bore receiving said piston rod and there being a sleeve surrounding said piston rod and having a conical bore. tapering inwardly around the piston rod toward said end wall to define a progressively restricted passage for actuating fluid leading from that chamber defined by said first piston member.

10. In a double acting servomotor of the type including piston-cylinder means having a pair of piston mem bers joined for unison movement by connection means and each having an associated chamber for receiving actuating fluid, at least one piston'rod connected to one of said piston members and control valve means connected to said chambers for differentially pressurizing them selectively to move the piston members in opposite directions and for holding the pressures in said chambers to position said piston members in selected fixed positions, the improvement which comprises:

bleed means for continuously circulating actuating fluid through both said chambers and past both said piston members irrespective of the operating condition of said control valve means; said piston members each being of frusto-conical shape having their smaller ends disposed upstream of the direction of actuating fluid flow effected by said bleed means whereby said piston members to the source.

11. In a double acting servomotor as defined in claim 10 wherein said bleed means includes a flow-restricting member in each circuit between the source of pressurized fluid and the associated chamber. 

1. A double acting servomotor comprising, in combination: an elongate cylinder having a bore of uniform diameter; piston means in said cylinder for defining first and second chambers therewithin, said piston means comprising a first piston member of frusto-conical shape having its larger end defining a restricted annular clearance space with said bore, a second piston member of frusto-conical shape having its larger end defining a restricted annular clearance space with said bore, and means joining said piston members to each other for movement in unison; a source of pressurized actuating fluid; control valve means selectively connecting said source to said chambers for differentially pressurizing them selectively to move the piston members in opposite directions and for holding the pressures in said chambers to hold said piston members in selected fixed positions; bleed means defining a fluid circulation flow path through each chamber and past each associated piston member for continuously conveying actuating fluid past each piston member from the small end to the large end thereof and to a bleed flow exhaust means and including means for continuously supplying actuating fluid to each flow path independently of said control valve means.
 2. A double acting servomotor as defined in claim 1 wherein said piston members are joined together with their larger ends in opposition and close proximity to define an annular space therebetween which is common to both said flow paths.
 3. A double acting servomotor as defined in claim 1 wherein said piston members are joined together with their smaller ends in opposition and in close proximity to each other to define an annular space therebetween which is common to both said flow paths.
 4. A double acting servomotor ay defined in claim 1 wherein said piston members are disposed in longitudinally spaced relation to each other and including a piston rod joining said piston members, a wall member closing said bore between said piston members and having a double conical bore receiving said piston rod, said double conical bore having a central annular space which is common to both of said flow paths.
 5. A double acting servomotor as defined in claim 2 wherein said bleed means includes a conduit leading to said annular space between the piston members for bleeding off actuating fluid therefrom and said means for continuously supplying actuating fluid including a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers and each having a restriction therein for metering the bleed flows of actuating fluid to such chambers.
 6. A double acting servomotor as defined in claim 3 wherein said bleed means includes a conduit leading to said annular space between the piston members for supplying actuating fluid thereto and a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers.
 7. A double acting servomotor as defined in claim 4 wherein said double conical bore tapers away from both piston members, and said bleed means includes a conduit leading to the central annular space of said wall member to supply actuating fluid thereto and a pair of conduits one connected to one of said chambers and the other connected to the other of said chambers for metering the blEed flows of actuating fluid from such chamber.
 8. A double acting servomotor as defined in claim 4 wherein said double conical bore tapers outwardly toward both piston members, and said bleed means includes a conduit leading to the central annular space of said wall member to bleed actuating fluid therefrom and a pair of conduits one connected to one of chambers and the other connected to the other of said chambers and each having a restriction therein for metering the bleed flow of actuating fluid to such chambers.
 9. A double acting servomotor as defined in claim 1 including at least one piston rod joined to said first piston member and projecting axially of said bore in the cylinder to pass outwardly therefrom, said cylinder having an end wall provided with a bore receiving said piston rod and there being a sleeve surrounding said piston rod and having a conical bore tapering inwardly around the piston rod toward said end wall to define a progressively restricted passage for actuating fluid leading from that chamber defined by said first piston member.
 10. In a double acting servomotor of the type including piston-cylinder means having a pair of piston members joined for unison movement by connection means and each having an associated chamber for receiving actuating fluid, at least one piston rod connected to one of said piston members and control valve means connected to said chambers for differentially pressurizing them selectively to move the piston members in opposite directions and for holding the pressures in said chambers to position said piston members in selected fixed positions, the improvement which comprises: bleed means for continuously circulating actuating fluid through both said chambers and past both said piston members irrespective of the operating condition of said control valve means; said piston members each being of frusto-conical shape having their smaller ends disposed upstream of the direction of actuating fluid flow effected by said bleed means whereby said piston members continuously are centered within said cylinder bore; and wherein said bleed means comprises conduit means connected to each chamber and defining with each chamber a closed circuit from a source of pressurized actuating fluid to the smaller end of the associated piston member and from the larger end of the associated pixston member back to the source.
 11. In a double acting servomotor as defined in claim 10 wherein said bleed means includes a flow-restricting member in each circuit between the source of pressurized fluid and the associated chamber. 